Protective circuits for electron tubes



' Nov. 4, 1941.

C. K. GIERINGER PROTECTIVE CIRCUITS FOR ELECTRON TUBES Original Fi1 ed Nov; '22,, 1938 3 Sheets-Sheet l all ma, Q 5622;;

INVENTOR.

BY- fi z /m g ATTORNEYS Nov. 4, 1941. I c. K. GIERINGER ,PROTECTIVE CIRCUITS FOR ELECTRON TUBES Original Filed Nov. 22, 1938 s Shets-Sheet 2 .90 I 60 K 1 29 usiag vnbvz i J: 70 Sfllzzrahon O 5 M I 1 1 do I $1: 2 W y 51 .Norinal 1251101158 N u x u n 1/ x g I 0 10 m 50 4o .50 so 20 a0 90 M0 021113112 232119 1527 5 OR f BY 4222 A TQBNEYdw Nov. 4, 1941. c. K. GIERINGER PROTECTIVE CIRCUITS FOR ELECTRON TUBES Original Filed Nov. 22, 1958 s sheet-sheet s INVENTOR.

M K ATTORNEYS Patented Nov 4, 1941 PROTECTIVE CIRCUITS FOR ELECTRON TUBES Carl K. Gieringer, Cincinnati, Ohio, assignor to The Liebel-Flarsheim Company, Ohio, a corporation of Ohio Cincinnati,

Refiled for abandoned application Serial No. 241,835, November 22, 1938. This application May 22, 1941, Serial No. 394,681

Claims.

This invention relates to electro therapeutics. The application is a continuation-in-part of copending application entitled Diathermy dosage meter, Serial No. 120,261, filed January 12, 1937. The copending application is directed particularly to improvements in the measurement and control of dosages of electrical energy applied to patients from short wave heat machines of the type adapted for use in physical therapy. The

present invention is directed to the protection of the machines themselves against damage arising from unbalanced electrical conditions in the circuits and tubes of them, and to the protection of the patients being treated against the appli cation of dosages exceeding those predetermined by the physician.

Broadly speaking, short wave physical therapy comprehends the generation of heat in the human tissues by means of electrical instrumentalities adapted to provide energy to the tissues in the form of rhythmic oscillating currents, which vary in direction of flow at highly frequent intervals. At the present time, short wave heat exposures are recognized to be of value in the treatment of infections in the respiratory tracts, in various suppurative processes, in chronic arthritis, in pneumonia and in the treatment of bursitis, and in similar instances where heat is expected favorably to influence physical conditions.

In general, two methods have been used in the past to induce short wave currents into the body or the parts of it which are afflicted.

The first of these is called the condenser method and involves the use of insulated spaced plates or pads. These plates or pads are associated with a short wave current generating apparatus, and, when a body of tissue is placed between them, the current flows through it by reason of its electrical capacity. During the passage (the body in a sense acts as a resistance) some of the electrical energy is converted or transformed into heat. This heat, in other words, is generated in situ. It is conventional to mount plates adjustably to facilitate the treatment, for example, 0f the head, knees, shoulders, ankles, etc., in instances where the pressure or contact pad is undesirable. For the most part pads are placed on either side of and in contact with the particular body sector to be treated.

The second method of applying short Wave currents to the body is known as the electromagnetic or-induction method. This method is employed particularly in fever therapy, and in the treatment of the internal parts of the body, or the parts which are irregular in shape. In

the induction method the heating is caused by the establishment and flow of secondary currents induced in the tissue being treated by reason of the current flowing in the applicator. The applicator usually comprises a cable or coil wound around the aillicted tissue.

In the diathermy machines either the spark gap or the electron tube has been employed as the primary means for causing the short wave oscillations. Both the present application and the copending application are directed to improvements in the electron tube type of generating apparatus.

1 While the electrical circuits and various electrical instrumentalities adapted to produce short wave currents have been understood quite fully in the past, still, it is recognized quite generally in the medical arts that the success of heat therapy in the treatment of ailments, where the technique is known to be of value, depends largely upon the skill and experience of the physical therapist or operator, as well as upon the regulated establishment of predetermined conditions.

Broadly speaking, the degree of heat established in a given mass of tissue in a patient by means of short wave currents applied to the body, either by the inductance method or by the condenser method is a function of. the power or energy supplied to the body. In other oscillating circuits, such as radio circuits, the coupling of the load to the circuit is substantially constant and in these machines the plate voltage or current furnishes a suiiiciently reliable indication of the power or energy being supplied from the circuit.

However, in short Wave therapy, even though the patient is insulated from the applicator pads or coils, there is a retro-active influence or loading by the patient circuit uponthe short wave oscillator with which the patient is coupled for treatment. This influence or loading is variable from patient to patient since the electrical characteristics of the tissues differ between individuals. The electrical loading upon this circuit also varies with the way a given patient is coupled to the circuit. Therefore, even when the same type of coupling its used, the load which is imposed upon the generator varies from patient to patient with other conditions being identical. In the treatment of leg tissues, for example, the load which is imposed upon the generator is variable from patient to patient, even when there is no variation in the relationship of the applicator elements to the leg. On the other hand, the load imposed during the treatment of the abdomen of a given patient is different from the load imposed when the shoulder, for example, is placed for treatment.

The problem of measurement of energy supplied to the part under exposure is complicated still further by reason of the fact that the load during the treatment of any of these various parts of the body varies in accordance with the relationship between the body parts and the applicator elements by which the energy is provided.

Thus, if pads be used, in contact with the parts, then the characteristics which influence the load upon the generator are not the same as the characteristics which influence the load when plates are used, or when a cord is wrapped around the mass of tissue in question. The nature of the mass of tissue, its size, the regularity or irregularity of its shape and surface, its water content and the degree of proximity of the applicator all comprise factors which affect the dosage received. It is not possible to measure the result by direct recording of the temperature even though the generation of a given degree of heat for a given period of time is the primary objective in heat therapy.

In the aforesaid copending patent application I have disclosed that the difference between the grid current and the plate current is a reliable indicator of the energy output to a patient. In the copending application, there is disclosed and claimed a circuit in which this current differential, between the average plate and average grid circuits, that is Ip-Ig, is employed for actuating a meter whereby the patient loading is measured and indicated.

The currents Ig and Ip are pulsating, not direct, currents, and therefore, the terms average plate current and average grid current are employed to designate the mean effective current which flows, since this average value is determined readily by measuring the current with a D. C. meter of the dArsonval type, Radio frequency components of the plate and grid currents are not involved. In the following specification, when plate current and grid current are referred to, it is the average or dArsonval meter measurements which are intended, unless otherwise described.

The present patent application is directed particularly to the employment of this differential value between the average plate current and the average grid current for the protection of the machines themselves against overloading and damage.

The proper functioning of a short wave oscillator depends upon a balance between the currents in the plate and grid circuits. Tubes may be damaged through overloading of the machine, or through such incorrect loading that a condition akin to overloading is imposed upon it, Either condition exerts a retro-active effect upon the oscillator which disturbs or tends to disturb its oscillation and alter the electrical conditions in the tubes.

For instance, when the tubes are oscillating, the grid current is relatively high, but it is relatively low when the circuit is not in oscillation. If the grid current is, for example, at a value of approximately 50 milliamperes and plate current 300, then no damage is done since under these conditions normal oscillation of the circuit exists. In such instance, the differential between the plate current and the high grid current is not unduly large. On the other hand, if the circuit is not oscillating efliciently because of some retro-active effect of the patient loading, then, even though the plate circuit may not be any higher than in the other instance, the grid current in this case is very low, or zero, and there is likelihood that the tubes will be damaged or overloaded because of the low efiiciency of the circuit and the attending abnormally high differential between the plate current and the grid current.

When these conditions occur the tubes may either function improperly or they become damaged permanently and need be replaced. A meter employed to indicate filament voltage cannot warn the operator of such unbalance and a meter either in the plate or grid circuit alone likewise is ineffective to indicate the dangerous conditions, In the past there has been no protection for the tubes, other than the precautions exercised in the original design of the circuit to meet normal conditions. These precautions, however, are wholly ineffective in relation to the wide variety of conditions under which the machines are employed, and in relation to the flexibility which is expected of them. Yet, the electron tubes in the circuits are very expensive to replace, and are so delicate that they are damaged quite easily by overloading.

The primary objective of the present invention has been to provide a circuit including a protective device which is responsive to the currents flowing in the grid and plate circuits, and which is responsive to the balance between them, and effective to limit and confine this value to a safe operating range.

The objective, in other words, has been to include in the generating circuit a protective device so constructed and arranged thatthe degree at which the electrical conditions in the tube circuits may become unbalanced is limited and controlled within a predetermined and safe operating value range. In this manner the tubes are protected against electrical damage.

A further objective of the invention also has been to employ metering means for measuring the amount of dosage, in conjunction with means for protecting the electrical circuits of the machines against damage, so that the patient, on the one hand, is protected against over-dosage, and the machine is protected against overloading, on the other,

A further objective of the present invention has been to provide a circuit including a meter which is responsive to the difference between the current of the plate and the current of the grid circuit, but which is so actuated by the difference between these currents that the deflection of the meter needle is proportionately greater for given load variations in the lower load ranges than at the loads approaching full load.

For the most part, the energy which diathermy machines are capable of delivering is variable over a substantial range and in most of these machines very few treatments are given at full load, or otherwise with very high energy output. Usually, the patient is treated to receive a relatively small quantity of energy for a relatively prolonged period. However, the machines also must be built to furnish high energy when its use is indicated by the nature of the patients afiiiction, and, consequently, most commercial machines are capable of operating over a wide range of power output. In most diathermy treatments the actual loading of the machine is within the range of A, to of the total power it is capable of delivering. In the more powerful machines the average local treatment may be about /5 to of the maximum available, for instance, Consequently, a meter reading directly proportional to the power output is deflected only a relatively small amount in the average diathermy treatment, and the rest of the scale is relatively little used. Obviously, it is much more diiiicult to read the meter when the deflection is slight, and it is likewise difficult to note the small variations which occur in the loading when the energy being received by the patient is relatively small. To obviate these difliculties, therefore, an objective of this invention has been to provide a meter which indicates the energy being received by a patient, but which is so actuated that the meter needle has a relatively large deflection for small variations in the lower energy ranges at which most of the treatments are conducted.

A still further objective of the invention has been to provide an annunciator in the generating apparatus that is effective to signal and warn of overload conditions, or otherwise to warn of the application of dosages exceeding those intended to be given.

The present invention, through which these objectives are satisfied, is predicated upon several concepts and discoveries. These are as follows:

Briefly, the present invention first is predicated upon the concept of employing the differential value between the plate current and the grid current as a primary control for the protection of the tubes themselves. The invention in this respect contemplates a circuit in which the plate current and grid current are applied. to a tube protective device that is responsive to them and capable of eifecting alteration of the electrical circuit conditions when these currents exceed a predetermined, safe operating differential.

In the preferred embodiment, the protective device includes a magnetic winding which is in such circuit connection that it is capable of generating a flux which is proportional to the diiier- .ence between the current of the plate and the current of the grid circuits. This magnetic flux is employed mechanically to actuate means which is effective to protect the tube. Such means may comprise a resistance or impedance which is normally out of circuit, but connected in the circuit to limit the value of the plate current when the differential exceeds the safe operating value, or a primary switch which disconnects the machine from a primary source of energy and sounds a warning that will enable the operator to correct the condition, or other means effective to prevent the impeding destruction or damage of the tubes. The warning device may be a separate buzzer or the relay may be arranged so that the armature chatters due to the pulsating current.

Next, the magnetic circuit which is effective for actuating the protective device for the tubes also includes a winding to which a meter is connected upon this meter the dosage being received by the patient is metered and indicated. As previously described, this indication is a reliable measurement of the amount of energy which the patient receives. Moreover, the magnetic circuit, or that part of it with which the meter winding is associated, is so arranged that its degree of saturation is not directly proportional to increases in the load, but rather so that the flux density increases or decreases rapidly for small load increases or decreases in the lower load ranges.

In the preferred structure the magnetic circuit generates a proportionately large flux in the lower load ranges at which most diathermy treatments are given, and consequently for small loads and for small load variations the deflection of the needle of the meter is correspondingly greater than in the ranges of the higher loads which are used infrequently. The net efiect of this arrangement is to spread the scale of the meter, that is, to make the meter easier to read in the ordinary therapy treatments. Upon the occasions when high energy output is required the meter still reads accurately.

The present invention has general application to shortwave oscillating circuits which are employed in heat therapy machines. For the purpose of assisting those skilled in the art in the practice of the invention and in the application of it to circuits of different types, its use is disclosed in a typical series fed oscillator, a typical shunt fed oscillator, which are both of simplified form, and with a shunt fed oscillator of the tune plate, tune grid type, in which two tubes are employed as in the circuits used in typical commercial machines. From the foregoing description of the principles of the invention and from the following disclosures and the embodiments of them in both simple and complex circuits of different types, the application of the invention to other cir'cuits readily will be understood by those skilled in the art.

In the drawings Figure 1 is a chart illustrating typical relative values of the plate and grid currents as the load of the machine is increased from no load to full load.

Figure 2 is a diagrammatic illustration of a shunt fed circuit employing a tank coil which is in shunt with the transformer and plate choke to the plate of the tube.

Figure 3 is a circuit similar to Figure l but it is series fed, the tank coil being in series with the transformer and plate of the tube.

Figure 4 is an illustration of the mechanical details of the protective device and the relative arrangement of the magnetically responsive parts of it.

Figure 5 is a chart illustrating the relative rate of change of flux generated in the magnetic circuit operating the load meter as the load is increased from no load to full load, whereby the meter scale is spread in the lower load ranges; and,

Figure 6 is a diagrammatic illustration of a typical tuned plate, tuned grid circuit of the type in common usage in commercial therapeutic machines.

SHUNT FED CIRCUIT THE OSCILLATOR The circuit of Figure 2 is comprised of a triode electron tube having a plate I, a grid 2, and a filament or cathode 3. The plate is in connection through the lead 4, with a terminal of the resonator coil 5. The opposite terminal of this coil is in connection through lead 6 with one of the terminals of the grid resonator I, the other terminal of which is in connection with the grid tube through the lead 8.

A transformer is employed for energizing this circuit. This transformer has a primary winding 9 which is in circuit connection with the source of energy, for example 60 cycle, volt current.

The transformer also includes a secondary winding l0 and a separate filament winding II. The

terminals of the filament winding are connected, through leads I2 and I3, to the filament of the tube. Intermediate these leads a pair of filament by-pass condensers I4 and I5 are connected, and a connection I6 is taken from a center tap between them to the lead 6 interconnecting the resonator coil and the grid resonator. In this circuit, radio frequency grid current flows through the grid resonator and radio frequency plate current flows through the resonator coil.

A tank condenser I1 is connected in parallel with the resonator coil 5. A blocking condenser I8 is connected in the lead 4 between the plate I and the resonator coil.

The patient circuit is inductively coupled with the resonator coil at the inductive coupling coil I9. The coupling coil is variable so that the patient circuit, i. e. the retro-active load imposed by the patient upon the oscillator, may be tuned with the oscillator for application of short wave current to the patient.

THE TUBE Pnornc'ron CIRCUIT The preferred tube protector circuit essentially is comprised of a circuit in which a, magnetic flux is generated by the current diiferential of the plate current and the grid current. This magnetic flux is employed mechanically to actuate a controller when the difference between the plate current and the grid current exceeds a safe operating value, whereby either the plate current automatically is limited or whereby other means are brought into operation to restore safe operating conditions or prevent further operation of the machine.

The winding in which the magnetic flux proportional to the difference between the plate current and the grid current is generated is indicated generally at 20. To one terminal of this winding is connected a grid return lead 2I from the grid resonator I. This lead also contains a fixed grid leak or resistance 22. In the lead flows the D. C. grid current, Ig. The other terminal of this coil 20 is connected through the lead 23, to the terminal of the secondary winding II] of the transformer, and the other winding of this transformer is connected, through lead 24, to the plate I of the tube. Therefore, in the lead 23 flows the direct current of the plate circuit, Ip.

A choke coil 24a in lead 24 blocks radio frequency current from the protector circuit.

Next, a center tap connection from the filament winding II of the transformer is taken, as lead 25, to a tap connection on the magnetic winding 20, but this lead 25 also includes a plate current limiting resistance 26 and a buzzer B. The resistance is shunted by the leads 21 and 28, which are taken respectively to a, contact switch 29. Switch 29 is actuated by an armature 30 which is spring weighted for movement by the flux generated in the coil 20 only when this flux exceeds a predetermined value.

Since the lead 2I carries the grid current and the lead 23 the plate current, the tap connection lead 25 carries the current Ip plus Ig or the current which is equal to the addition of the plate current and the grid current. The contacts 29 normally are closed and Ip+Ig current is free to flow through the lead 21 and back to the magnetic winding, by-passing the resistance 26. However, the armature is so weighted and adjusted that, when the magnetomotive force produced by 11) and Ig in their respective windings exceeds a predetermined safe operating value for the tubes, then the flux generated in coil 20 is sufficient to actuate the armature and it moves to open the switch 29 and, thus, to include the resistance 26 in the circuit. This resistance is so proportioned that it is effective to reduce the plate current and prevent tube overloading. The buzzer B is connected in the circuit to enunciate this condition when it occurs.

A meter winding 3| also is magnetically associated with the core around which the magnetic coil 20 is wound and this winding 3| is employed to operate the meter 32 for indication of the energy being received by the patient.

The chart of Figure 1 illustrates the relative values of the plate current and the grid current at varying loads, from no load to full load. For any given load, the difference between the plate current and the grid current is represented by the length of the vertical line drawn from the plate current curve to the grid current curve. The chart also illustrates the typical instance in which the differential between plate current and the grid current exceeds a safe operating value. These curves, of course, vary from circuit to circuit, but generally approximate the paths of the curves shown in Figure 1.

METER SCALE SPREAD Since the flux generated in the magnetic winding is proportional to the difference between the plate current and the grid current, the needle of this meter also is deflected in proportion to the difference, and therefore, this meter serves to measure the amount of short wave energy being received by the patient.

As described in the aforesaid copending application, Ig and I1: currents may be applied directly to the meter so that it will be deflected in accordance with the difference between them, with a resistance being interconnected between the l and Ig leads in place of the magnetic winding 20, and with the IpIg lead (i. e. lead 25) being connected to a tap on the resistance. Variation of the tap location upon this resistance serves to set the meter reading at zero when there is no load upon the machine.

However, in the circuit shown in Figures 1, 4 and 5 the magnetic winding 20 serves both as a reactance and resistance and the Ig and 1 portions of this winding on opposite sides of the tap are proportioned in respect to one another so that the flux generated in the winding 3I causes the meter to read no load when there is no load upon the machine.

The windings 20 and 3| are wound around a magnetic core of which the mechanical details are illustrated in Figure 4, and the function in Figure 5. This core is comprised of arm portions 3333 and cross-pieces 34 and 35, with the winding 20 and the winding 3| being carried on the arm 34. The core is so designed that the cross-piece 35 and the lower portions of the arms 33 contain less iron than the upper portions of the arms and the cross-piece 34 around which the core is, wound, for example, by omitting alternate or"'complementary laminations at these portions. Therefore, the lower portion of the core which is designated A is less permeable than the other portions, and the flux which is generated in the core is of highest density at the portions adjacent the armature 30. The core is preferably designed so that the iron is saturated when the machine is operating at less than full load, for instance, at about half load. Under these circumstances, the saturation of the iron increases rapidly from no load on the machine to approximately half load and the rate of increase then gradually becomes smaller as the machine is loaded beyond the half load point, as shown in Figure 5.

Consequently, in the lower working ranges of the machine there is, relatively speaking, a much greater flux density in the magnetic circuit and a much greater variation as the load varies, than as the load becomes greater. Thus, as the load upon the machine is increased from no load to full load, the rate of deflection of the meter is rapid up to about the half load point, and becomes less rapid beyond this. The metering circuit is most sensitive in the range where most of the diathermy treatments are given, and still gives true indications of energy being received by the patient even in the full load range.

SERIES FED CIRCUIT The circuit of Figure 3 is a series fed circuit; it includes a triode electron tube having a plate, a grid and a filament as in Figure 2. The grid resistor and the resonator current are in the same respective circuit connection as in Figure 2, and likewise, the filament is supplied with energy from the filament transformer and the patient circuit also is coupled in the same manner. However, in this circuit the blocking condenser l8 of the circuit of Figure 2 is omitted, the resonator or tank coil is connected in series with the terminal of the secondary winding of the transformer through the lead 35, and a blocking condenser 3! is inserted in the lead 6.

In this circuit the grid resonator 1 carries radio frequency grid current and the resonator coil 5, being in series with the secondary of the transformer, carried both D. C. and radio frequency plate current. The circuits of Figures 2 and 3 otherwise are the same, including the tube protector and magnetic circuit portions of them, and the same numerals are therefore applied.

CIRCUIT OF TYPICAL COMMERCIAL MACHINE (a) Plate circuit Through the leads 42 and 43, the plates of the tubes are in series circuit connection with the respective terminals of the secondary winding as of a transformer which is indicated generally at '55. The secondary winding coil has a center tap 46 of which the function is described at a later point in the specification.

The leads 42 and 63 also contain choke coils 41 m respectively. These serve the purpose of preventing high frequency currents from entering the secondary winding of the transformer.

Plates 39 are shunted through the leads 48 and t9, the former containing sets of condensers 58-59, and the latter, 5l5i, both sets of which prevent the entrance of low frequency current from the transformer into the other parts of the oscillator.

The transformer 45 is comprised of a core 52 around which the secondary 44 is Wound. This core also carries a primary winding 53 which is in circuit connection with a source of power, (for example. 60 cycle, 110 volt current) through the leads 5' 1 and 55, and a filament winding 56 for energizing thev filaments of the. tubes. The source lead-55 contains a switch 51 having a plurality of contacts which are in'a tap connection with the primary winding for adjustmentv of the transformation of, voltage, and both leads 5 and 55, contain choke coils 58 respectively whose functionis similar to that of the choke coils 41.

(b)- Grid circuit (c) Filament circuit The terminals of the filaments ll4l of the tubes are interconnected to each other through the leads lid-64, and these in turn are cross connected through lead 65 which contains a pair of condensers 6666. From a tap intermediate these condensers, a lead 6! is taken to the'terminal lead 6| of the resonator coil 621 In this manner the grid and plate circuits are associated at the resonator coil in which their currents oscillate. Energy is supplied to the filament circuit from the filament transformer winding 56, through the respective leads 68 and 69;

(d) Patient circuit The patient circuit is comprised of a fixed coupling coil 10 and a variable coupling coil 1 I, both of which are inductively coupled to the tank or resonator coil 62.

The fixed coupling coil circuit includes a variable condenser 12; this is connected in series with the coil and outlet terminals, 13 and I4, and also to outlet 75 through inductance coil 16. In this circuit inductive coupling to the oscillator is fixed, and the capacitance is variable to suit the treatment requisites. This circuit constitutes the electro-magnetic field circuit for patient treatment and it is adapted particularly to supply energy suitable for treatment by theinductance method which is described in the forepart of this application. For example, acable which is wound around the body part of tissue to be treated is connected across the terminals Hand 14. The inductance coil Hivaries the current characteristics to permit the use of another type of cable.

The electro-static field for treatment of patients by the condenser method is supplied from the variable coupling coil H. The terminals of this coil, respectively, are connected to outlet terminals l1 and 18, or through the condenser 19 to the outlet terminals 11 and 80. Applicators such as pads or plates are connected across the terminals 1'! and 18 or 80. This particular circuit also is adapted to provide energy suitable for electro-surgical sections. When cutting or coagulating current is desired, the appropriate scalpels may be connected across the terminals H and or H and 18; however, in this latter instance, the terminal 17 is also connected through the switch 8| and the blocking condensers am back to the filament lead 68.

TUBE PROTECTOR Cmcmr From the lead 8| interconnecting the grid oscillator excitation coil 60 and the tank or resonator coil 62, a grid return lead 82 is taken to a terminal of the electro-magnetic winding of the protecting device, indicated generally at 83. The other terminal of this winding is in connection, through lead 84, with the tap 46 of the secondary winding of the transformer 45. The grid current Ig fiows through the grid return lead 82. The plate current I]: flows from the secondary of the transformer through the lead 84, and these currents are so applied at a winding 83 that they buck one another and so that the flux which is generated in the coil 83 is proportional to the difference between them. The grid return lead 82 also includes the grid bias resistance 85.

Next, from a tap on the coil 83, intermediate its terminals, a lead 86 is taken to a tap on the filament winding 58 of the transformer 45. This lead 86 includes the contact switch which is indicated generally at 81. As previously described, this contact switch normally is closed and the armature (not shown in this circuit) which is associated with the coil 83 is arranged to open this switch when the flux generated by the difference between the plate current and the grid current is of sufiicient density to actuate the armature.

The armature, as previously described, is so adjusted that it is actuable by the fiux which is generated when the difference between the plate current and the grid current exceeds a safe operating value.

A current limiting resistance 88 and a buzzer or annunciator 89 are connected in series through lead 98, and these elements are shunted across the protecting switch 8! through the leads 9 I, 92 and 93. When the switch 81 is closed, current does not flow through the resistance 88 nor the buzzer. However, when the armature of the protecting device is actuated to open the switch, then current is caused to flow through the annunciator to sound the signal, and the resistance 88, to limit the current difierential.

METER CIRCUIT A winding 94 is associated with the electromagnetic coil 83, for energizing the meter which is indicated generally at 95. However, in this circuit the meter also is employed alternatively for indicating filament voltage. The meter normally is in circuit connection to indicate the difference between the plate current and the grid current and so to indicate the amount of energy being received by a patient undergoing treatment. Alternatively, it is connectable in the circuit, as

by pressing a push button switch, to indicate the the filament transformer and the other terminal of this coil is connected through the lead 96 and the fixed resistance 91 to a terminal 99 of the meter, through a push button switch which is indicated generally at 88. The other terminal I88 of the meter is connected through lead IM to the lead 92. A by-pass condenser I82 is connected across the terminals 99 and I88 of the meter to prevent radio frequency current from entering the metering circuit.

With these circuit connections to the meter, it normally reads the difference between the plate current and the grid current. By adjusting the relativ position of the take-off of lead 86 upon the electro-magnetic coil 83, the amount of flux generated in the coil 94 is adjusted so that th meter reads no deflection at no load when the oscillator is in operation.

The meter 95 also includes a filament winding which is connected across the terminals I83 and I84. A blocking condenser I85 similar in function to the blocking condenser I82 is connected across the terminals. The terminal I84 is connected through lead I86 to the remaining contact of the push button switch and the terminal I83 is connected through lead I81 to the filament lead 69 and so to the filament transformer. A pilot light I88 is connected across th leads IM and I81, but with a blocking condenser I89 parallel with it. This pilot light is energized when th oscillator circuit is in operation.

This application is a re-filing of copending ap plication Serial No. 241,835 filed November 22, 1938.

Having described my invention, I claim:

1. An oscillating circuit including an electron tube having a grid element and a plate element coupled for oscillation, a source of energy for oscillating the circuit, electro-magnetic means in circuit connection with the grid and the plate of the tube and actuable in response to the difierence between the currents flowing in the said plate and grid when the circuit is being supplied with energy for oscillation, the electro-magnetic means being normally inactive but actuated when the difference between the plate current and the grid current exceeds a predetermined value, and a device which is responsive to actuation of the said electro-magnetic means, including a current limiting impedance in connection with the plate of the tube for limiting the sum of the plate current and the grid current to a predetermined value.

2. An oscillating circuit including an electron tube having a grid element and a plate element coupled for oscillation, a source of energy for oscillating the circuit, a magnetic coil in circuit connection with the grid and the plate of the tube for generating a flux which is related to the difference between the currents flowing in the plate and the grid of the tube when the circuit is being supplied with energy for oscillation, and means including a current limiting impedance in the oscillating circuit and a switch which is actuable by the fiux generated in the said coil when the flux exceeds a predetermined value, the switch effecting circuit connection of the current limiting impedance in the oscillating circuit for limiting the sum of the plate current and the grid current to a predetermined value.

3. An oscillating circuit including an electron tube having a grid element and a plate element, a source of energy for oscillating the circuit, a coil in circuit connection with the grid and the plat of the tube for generating a flux which is proportional to the difference between the currents flowing in the plate and the grid of the tube when the circuit is being supplied with energy for oscillation, and a circuit inductively coupled to said coil and containing a meter for indicating the amount of fiux generated in said coil,

the meter thereby indicating the amount of power being supplied from the oscillating circuit when it is in oscillation.

4. An electro therapeutic machine comprising an oscillator including a vacuum tube having a plate, a filament and a grid, said oscillator comprising a circuit between said plate and said filament, and a second circuit between said grid and said filament, said circuits having a portion in common, an impedance device in said common portion, and means for producing a controlling effect which is proportional to the difference between the direct components of the plate and grid currents, and means for exerting said controlling effect to render said impedance efiective upon said difierence exceeding a predetermined value.

5. An electro therapeutic machine which comprises an oscillator including a vacuum tube having a plate and a grid and a filament, a circuit between the plate and the filament, a second circuit between the grid and the filament, said circuits having a portion in common, a circuit, limiting impedance in said common portion, means including a coil connected between the grid and the plate of said tube for generating a magnetic flux proportional to the difference between the plate current and the grid current, and means controlled by the flux generated by said coil for rendering said impedance effective for limiting the sum of the plate current and the grid current to a predetermined value.

6. An apparatus for producing oscillating current which comprises an electron tube having plate, cathode and grid elements, a circuit between said plate and said cathode, a second circuit between said grid and said cathode, said circuits having a portion in common, a current limiting impedance in said common portion, means including a switch for shunting said current limiting impedance, and a conductive circuit interconnecting said plate and said grid and including means for operating said switch when the difference between the grid current and the plate current exceeds a predetermined value to render the current limiting impedance effective for limiting the sum of the plate current and the grid current to a predetermined value.

7. An apparatus for producing oscillating energy which comprises an electron tube having a grid, plate, and a filament, a circuit between the plate and the filament, a second circuit between the grid and the filament, said circuits having a portion in common, a current limiting impedance in said common portion and adapted to be rendered efiective to limit the sum of the plate current and the grid current, and conductive means connected between the plate and the grid of said tube for rendering said current limiting impedance effective to control the sum of the plate current and the grid current upon the difference between the plate current and the grid current exceeding a predetermined value.

8. An oscillating current including an electron tube having grid, plate and filament elements, a circuit between the plate and filament of the tube, a second circuit between the grid and the filament of the tube, said circuits having a portion in common, said common portion including a current limiting impedance, a coil in conductive circuit connection between the plate and the grid of the tube for generating a flux proportional to the difierence between the plate current and the grid current, and means including a relay device responsive to the flux generated by said coil for shunting said current limiting impedance.

9. An electrical therapeutic machine comprising an oscillator including a vacuum tube having a plate, a filament and a grid, said oscillator comprising a circuit between the plate and the filament and a second circuit between the grid and the filament, a control device arranged when operated to prevent the tube from being operated at unsafe electrical values, means for producing a controlling effect which is proportional to the difierence between the direct components of the plate and grid currents, and means for exerting said controlling efiect upon said control device to render said control device efiective.

10. An electrical therapeutic machine which comprises an oscillator including a vacuum tube having a plate, a grid and a filament, a circuit between the plate and the filament, a second circuit between the grid and the filament, a control device arranged when operated to limit the electrical energy supplied to the vacuum tube, means including a coil connected between the grid and the plate of said tube for generating a magnetic flux proportional to the difference between the plate current and the grid current and means controlled by the flux generated by said coil for rendering said control device efiective.

CARL K. GIERINGE'R. 

